Epidemics of organic mercury poisoning in Japan, Iraq, the United States, and very recently, Canada, have indicated that the human fetus is at considerable risk for neurologic damage. We are using the monkey (Macaca mulatta or M. arctoides) to examine various aspects of exposure to methylmercury including: a) transplacental passage on chronic and acute bases, b) minimal toxic maternal intake throughout gestation which results in observable motor and mental retardation in the neonate, c) uptake and retention by different tissues, and d) localization and neuroanatomical damage in various brain regions. In humans, body burdens of mercury derived from environmental sources are being surveyed in a middle-class, Midwestern population of mothers and their infants. Tissues being assayed by atomic absorption spectrometry for organic and inorganic mercury include maternal blood and breast milk samples, neonatal cord bloods and placentae.